1. Field of the Disclosure
The present disclosure relates generally to apparatus and methods for determining a closure pressure of a fractured formation.
2. Description of the Related Art
During both drilling of a wellbore and after drilling, fluid (oil, gas and water) from the formation is often extracted to determine the nature of the hydrocarbons in hydrocarbon-bearing formations. Fluid samples are often collected from formations at selected wellbore depths by a formation testing tool conveyed in the wellbore. The collected samples are analyzed to determine various properties of the fluid. Some formations, such as made of shale, have very low permeability (also referred to as “tight formations”) and do not allow the formation fluid to flow into the wellbore when such formations are perforated to recover the hydrocarbons therefrom. Fractures, also referred to as micro-fractures are created in such formation to determine a geological characteristic of such formation. A useful characteristic or parameter of such formations is the closure pressure.
To determine the closure pressure in tight micro-fractured formations, a flow-back test (a test that involves flowing back the fluid from the fractured formation) can be used to determine the closure pressure of the formation. A deflection point in the pressure measurements made during the flow back test can be used to determine the closure pressure. During flow-back tests, it is desirable to draw the fluid from the formation into a testing tool at a constant or substantially constant flow rate. Such constant flow rates can be achieved by creating a positive pressure difference between the formation and a chamber in the tool receiving the fluid. Conventional formation testing tools are difficult to use for flow-back tests because such tools utilize reciprocating pumps, which pumps create a negative pressure between the formation and a receiving chamber in the tool. In addition, the reciprocating “strokes” of such pumps creates back pressure, which can obscure the clear identification of the deflection point in the pressure during the withdrawing of the fluid from the formation, which can lead to a large error in determining the closure pressure.
The disclosure herein provides an apparatus and method for determining the closure pressure of a fractured formation using a flow back test.